As a conventional fish finder (underwater detection apparatus), an apparatus having a function to estimate a length (size) of a fish (target) is known (see for example “Barange, M., Hampton, I., and Soule, M. 1996. Empirical determination of in situ target strengths of three loosely aggregated pelagic fish species.—ICES Journal of Marine Science, 53: 225-232”, “Sawada, K., Furusawa, M., Williamson, N. 1993. Conditions for the precise measurement of fish target strength in situ.—Ocean Acoustic Journal, 1993, Volume 20, No. 2, p 73-79”, “Gauthier, S., Rose, G. 2001. Diagnostic tools for unbiased in situ target strength estimation.—Canadian Journal of Fisheries and Aquatic Sciences, 2001, 58, p 2149-2155”). Typically, with a fish finder, when a peak waveform (which may also be referred to as a peak) of a receive signal obtained from a reflection wave reflected on a fish satisfies given conditions, the peak waveform is considered as being caused by a single fish and a single fish is detected. Specifically, with a fish finder, a single fish is detected based on various information obtained from a peak waveform such as peak intensity of the peak waveform, shape of the peak waveform, phase stability information etc. Then, with a fish finder, the peak intensity of the peak waveform of the detected single fish is transformed into a TS (Target Strength) value and a fish length is estimated based on said TS value.